Plasma process control in a plasma reactor typically relies upon a feedback control loop governing the RF power generators in response to plural RF sensors at different locations in the reactor. For example, RF power of different frequencies may be applied through one or more RF impedance matches to an electrode in the workpiece support. In such a case, one or more feedback control loops governing the RF generators or RF impedance matches may respond to RF sensor devices placed at the inputs and outputs of the RF impedance matches. Respective RF sensors may be provided for the respective RF power frequencies at the inputs and outputs of the impedance matches. In one example, two RF power frequencies are applied to the workpiece support electrode, so that four RF sensor devices are employed, one for each of the two frequencies at the impedance match inputs and outputs. Each RF sensor device may be capable of measuring RF parameters such voltage, current and/or phase.
A problem arises if the applied RF power is pulsed. During each pulse duty cycle off-time, the applicable RF sensor device reports a zero RF power level, which introduces an instability into the feedback control loop governing the respective RF power generator. The problem is complicated on a further level because the RF power of the different RF frequencies may be pulsed at different pulse frequencies (pulse repetition rates) and with different duty cycles. The non-zero measurement values obtained during the pulse duty cycle on-times are needed by the feedback control loops, but the zero measurement values obtained during the pulse duty cycle off-times are false and moreover introduce instabilities into the feedback control loops. A simplistic approach of merely suppressing all zero measurements would be unworkable because it would suppress those measurements that have a true zero value during a pulse duty cycle on-time, thereby introducing errors. One difficulty is how to distinguish between false zero measurements attributable to the pulse duty cycle off-times, and a true measurement of zero. There is an unfulfilled need to overcome the foregoing problems while allowing for the different pulse parameters at the different RF sensor devices and while concurrently capturing measurements from all the RF sensor devices at sufficiently high sampling rates required by the feedback control loops governing the RF power generators or RF impedance matches.